Zinc cyanide electroplating bath and process



United States Patent 3,296,105 ZINC CYANIDE ELECTROPLATING BATH AND PROCESS John D. Rushmere, Grand Island, N.Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Apr. 3, 1964, Ser. No. 357,291 17 Claims. (Cl. 20455) This invention relates to the electrodeposition of zinc and, more particularly, to the electrodeposition of zinc from alkaline cyanide plating baths whereby bright deposits are obtained.

Solutions for the electrodeposition of zinc are broadly classified into two general categories, namely, acid zinc baths such as those containing zinc sulfate, chloride or fluoborate, and alkaline zinc baths which contain zinc cyanide. Additives which are suitable for one category are seldom suitable for use in the other type of bath. The mode of plating is further classified into two groups, i.e., still plating and barrel plating. In both groups, electrodeposited zinc generally has a dull appearance unless a brightening agent is added. A wide variety of such agents have been'successfullyused; however, most have not been fully acceptable for all types of plating. Some work well for still plating while others are only suitable for barrel plating. In addition, some are extremely expensive or must be used in large quantities and others suffer from instability in the plating bath. Others yield bright deposits but the deposits are not responsive to post-treatment with protective agents.

It is, therefore, an object of this invention to provide a plating composition which will impart a uniform brightness to metal deposited from a zinc plating bath using either still or barrel plating techniques. It is another object to provide a brightening composition which is stable in storage and in the cyanide plating bath. A further object is to provide a plating bath which produces bright zinc deposits that are responsive to post-treatment with protective agents. Still another object is to provide an improved process for electrodepositing bright zinc. These and other objects will be apparent from the detailed description which follows.

The objects of this invention are accomplished by the use of selected amine oxide as brightening agents in an aqueous cyanide zinc plating bath. The effective amine oxides which contain a EN 0 group. are soluble in the plating bath and have reduction products, i.e., the parent amines which are formed by removing the oxygen from the EN+0 group, which are also soluble in the bath. If the parent amine tends to 'be absorbed onto the surface to be plated, introduction of groups which further increase -the solubility of the parent amine in the plating bath may be necessary. By the term soluble it is meant having a solubility of at least about 0.1 gram per liter at a temperature of about 25 C. in an alkaline cyanide zinc plating bath.

Compounds which meet the aforementioned requirements include both saturated and unsaturated organic amine oxides. Amine oxides which are substituted with solubilizing agents such as COOH, OH, and SO H groups are generally preferred to the unsubstituted compounds. Other groups such as halogen, amino, cyanide, and nitro groups may be present. Included among the suitable compounds are trimethylamine N-oxide, nicotinic acid N-oxide, m-dimethylaminophenol N-oxide, methylnicotinate N-oxide, pyridine N-oxide, isoquinoline S-sulfonic acid N-oxide, and nicotine-1' N-oxide. Of these representative compounds, nicotinic acid N-oxide is a preferred species.

The amine oxides may be used alone in the usual commercial plating baths but are preferably used in combina- See tion with an organic polymer which is also soluble in the bath. In fact, when the two components are combined, the brightening effect is not merely additive but rather a synergistic effect is observed. Examples of suitable polymers include synthetic polymers such as polyvinyl alcohol and polyethyleneimine, and natural polymers such as gelatin and peptone. The polyvinyl alcohol may be modified, e.g., a partial ether formed by condensation with-ethyleneoxide or glycidol.

The polyvinyl alcohols prepared by alcoholysis (hydrolysis) of polyvinyl acetate are a preferred type of organic polymer. Those having molecular weights from about 5,000 to 20,000 when used in combination with the N-oxide exhibit outstanding toleranceto sodium carbonate which builds up in the plating bath. This carbonate is formed in the bath by adsorption of carbon dioxide and electro-oxidation of cyanide. Asthe carbonate level increases, there is a tendency for the polymer to precipitate with resultant inferior plating. The combination of nicotinic acid N-oxide with a partially hydrolyzed (87-89%) grade of polyvinyl alcohol having an average molecular weight of about 16,800 has been found to be stable in baths containing as much as 12 ounces per gallon of sodium carbonate.-

The amine oxide brightening agents of this invention may be used in conventional aqueous cyanide zinc plating baths in amounts ranging from about 0.1 gram per liter to an amount approaching saturation. Optimum amounts will vary, depending on the specific compound used, the current density employed, and the type of plate desired. It is generally preferred. that from 0.5 to: 10 grams per liter be used. When used in combination with an organic polymer, the amount of the amine oxide should be in the range from 0.1 to 10 grams per liter, with from 0.02 to 10 grams per liter of the polymer being present in the bath. Preferably 0.05 to 0.5 gram per liter of the polymer is used.

Inone embodiment of this invention, a plating bath containing zinc cyanide, sodium cyanide, sodium hydroxide, an amine oxide, and a bath-soluble organic polymer is prepared by dissolving the constituents in water. The N-oxide and organic polymer may be added separately but are preferably added together in-the form of a concentratedaqueous brightener solution; This solution may contain from 1:1 to 50:1 parts by weightof N-oxide to polymer. Preferably, the solution contains from 3:1 to 9: 1, parts by weight N-oxide, to polymer. If the. amine. oxide is in acid form, it may be convertedto the sodium salt by addition of sodium hydroxide. This solution can i c more readily formulated in concentrated form. The concentrate may be diluted to provide an aqueous solution containing from about 10% to 35% solids;

The invention will be further illustratedby the following examples in which parts and percentages are by weight unless otherwise specified.

EXAMPLE I In a series of experiments, an aqueous plating bath, having the following composition, was used to plate automobile crank hole covers:

Grams per liter Sodium cyanide 40 Sodium hydroxide 90 Zinc cyanide 60 The crank hole cover, which had a total surface area of sq. ft., was suspended as the cathode in the bath and plated between two zinc anodes for fifteen minutes. At the end of this time, the crank hole cover was removed from the bath, washed in water, dipped briefly (two to five seconds) in 0.2% by weight nitric acid solution, rewashed in Water, and dried. (The crank hole cover which was used in the electroplating tests has an outer surface comprised of a series of concave surfaces and an inner surface comprised of a series of convex surfaces.) In the experi- Grams per liter Sodium cyanide ments, various amine oxides were added in the amounts Sodium hydroxide 75 indicated in Table 1, which follows: Zinc cyanide 60 Table 1 Experiment N-Oxide Amount Added Current Results Observed (g-l (amps) 1 Control None 3. 5 All surfaces were generally dull with little brightness or luster.

2 Trimethylamine N-oxide 1. 3.5 All surfaces much brighter than control.

3 Nicotinio acid N-oxide 1.55 3.5 Excellent deposit with good color, brightness and uster.

I C O OH N J. O

4 Nicotine-1 N-oxide 4.65 3. 5 Bright deposit with good color on concave surfaces.

Cs \N 3 O 5 Pyridine N-oxidc 10.8 7.5 Conca ve surfaces were bright and had good color.

Convex surfaces improved over control.

N -L O G 2,6-lutidene N-oxide 1. 55 7. 5 All surfaces had good color, brightness and luster.

CH3 l CH; N -L O 7 Isonicotinie acid N-oxide 7. 7 3. 5 Excellent blue-white deposit having good brightness and luster on all surfaces. CODE 8 N-methylmorpholiue N-oxide 1. 55 2. 5 All surfaces brighter than control.

EXAMPLE II The plating composition was agitated with activated A series of experiments were conducted using a conventional Hull type plating cell and an aqueous cyanide plating bath having the following composition:

carbon to remove traces of impurities and filtered through a fritted glass filter. Polished brass cathode panels were used in the plating tests. While substantially identical results were obtained using steel panels, because of the highly polished nature of the brass panels, ditferences in plating characteristics were more readily discernible.

In conducting the experiments, 267 ml. of the plating solution was placed in the Hull cell and various amounts of different amine oxides were added. The electrolysis was conducted at room temperaure (about 26 C.) using a current of 2. amps. for ten minutes. The cathode panel was then removed from the cell, washed in water, dipped in 0.5% by weight nitric acid for about twenty seconds, rewashed in water, and dried.

By using the Hull cell, a current density range from 2 to 150 amps. per sq. ft. is obtained across the plate. All plate obtained from baths to which an amine-oxide was added showed significant improvement over a blank. test Table 2 Amt.

Observed Current Density Ranges (Amps. per sq.- It.) For- Experi- N -Oxide Suhstitueuts, Position, Group Added, ment and Formula g./l.

Brilliant Bright Hazy Dull Plate Plate b Plate e Plate d 9 Control None- 2-150 10 Pyridine N-oxide 1. 9 10-30, 30-150 30-40 2-10 11 Pyridine N-oxide 5. 7 40-150. 10-10 2-10 12 -nitropyn'dine 0.38 10-80 2-10, 80-150 N-oxide NO N)O 1. 12 2-150 13 2-picoline N-oxide 2, CH3v 5. 7 i 10-25, 80-150 25-80 2-10 14 3-picoline Noxide 3, -GH 5.7 A 20-40- 40-150, 10-20 2-10 15 4-pic0line N-oxide 4, CH3 1.9 10-100 2-10, 100-150 16 2,6-lutidene N-oxide. 2,.CH3 6, CH3. 3.8 10-40 2-10, 40-150 17 Z-picolinic. acid N-oxide v '2, -'-C OOH" 1. 9 40-150 5-40 2-5 18 3-picolinic acid Noxide .3, +0 0 0H] 1. 9 8-150 z-s (Nicotinic acid N-oxide) 5. 7- 30-150 10-30- 2-10 19 4-picolim'c acid N-oxide 4, -o 0 OH 1. 9 5150 2-5 (Isonicotinic acid N-oxide) 5. 7 10-60 60-150 2-10 20 2,6-dicarboxypyridine Noxide 2, C O OH 6, C O OH 5. 7 15-150 5-15 2-5 21 4-cyanopyridine N-oxide 4, CN 1. 9 5-100 2-5, 100-150 22 Methylnicotinate N-oxide 8, 0 O 0 CH3 1. 9 70-150 10-30 2-10, -70

23 3-pyn'dylcarbinol N-oxide 3, CH2OH 1. 9 5-150 2-5 24 4-methoxypyridine N-oxide 4, CH3O- 5.7 60-150 15-60 2-15 25 Trimethylamine N-oxide (CH3) 3N O 5. 7 5-100 2-5, 100-150 I 20 m-dimethylaminophenol Q-monm o 5.1 7-50 2-7, 50150 N-oxide 27 Nicotine-1' N-oxide I N 5. 7 -150 20-40 2-20 \a N C a 0 28 Nicotine-1,1 N-dioxide 1. 9 -150 I 30-150, 2-10 10-30 N 5. 7 30-60 2-30 CH3 0 CH 29 N-methyl morpholme N-oxide 0 N 1. 9 10-35 0 ?H3 ('3 O OH 30 Quininic acid N-oxide 0.11 15-100 2-15, -150 0.38 35-150 2-35 'See footnotes at end of tables.

Table 2C0ntinued Observed Current Density Ranges Amt. (Amps. per sq. it.) For- Experi- N-Oxide Formula Added, inent g./1.

Brilliant Bright Hazy Dull Plate I Plate Plate Plate 6 31 Quinolinc -carboxylic acid 0 O OH 0.38 -30 2-10, 150

N oxide.

32 Isoquinoline s-sulfonic acid 0. 75 10-30 2-10, 30150 N-oxide. 1. 9 -150 2-8 5. 7 2-8 I Bright and lustrous plate having a mirror-like quality. b Bright and lustrous but less brilliant than (a). o Bright and lustrous background but slightly overcast with dull, gray-white haze. Flat gray-white, lacking luster and brightness.

EXAMPLE IV Grams per liter A series of experiments was conducted foll wing the ggfgt g'f 3g n rocedure described in Exam lo I and the sun Platl g P P 30 Zmc cyamde barrel plating procedure of Example H. A current of 5 amps. was used for the barrel plating and 3.5 amps. for In part of the experiments, 60 grams per liter of sothe still plating. In the barrel plating experiments, dium carbonate was added to the bath to illustrate the grams of No. 6 x inch screws were plated. A crank tolerance of the polymer additive. The .two types of hole cover was used in the still plating experiments. The 35 bath will be referred to as Standard and Carbonate" cyanide plating "bath had the following composition: baths in Table 3 which records the results of the experiments.

Table 3 I Additives (conc. g./l.) Experiment T y petof Bath Used Results Observed N-Oxide Polymer Ban-e1 s -m Screws were medium gray; matte appearonce, no brightness or luster. Nlcotlmc, 1 5 /1 Improvement over blank test. Screws were whiter in color, smoother, had v I developed some luster. Elvanol 71-24, I 0.075 g./l- Screws hadgood luster but were covered with en ob ectionable yellow-brown film. N t 1,5 /1 Evanol 71-24, 0.075 g./l Excellent product; screws had good colors I brightness and luster. Carbonam Nicotnnc, 3.0 g./l "Elvanol 51-03," b 0. 95 g-l Very g d product; screws had good color on us er. Still do "do E ol 51-03," 0.195 g./1 Erlrcelent product; good plate on all suraces. B L Nicotinic, 1.5 g./l GEPVA (3%). 0.075 g-/l-- V ry g od pr du t; screws had good color,

luster and brightness. Sti1l -do Nicotmic, 0.825 g./l GEPVA (6%), 0.075 g./l Ggod product; nice bright zinc on allsuraces. e d Nicotinic, g-l HEPVA,d 0. all Good product; very bright blugwhite I color, good luster. do Polyethyleneimine, 0.075 g./l--- Very good product; good color, brightness,

luster; good as nicotinic acid 01 alcohol combination. p yvmyl Carbonate" Ni otinic, ,75 g,/1 Gelatin, 0.38 gJl Reasonable p ct but ge a y inferior to polyvinylalcohol combination. "m" stanglardfln Pyndme, 13 gJL d0 sclrewithad good appearance, color, and

rig ness. "Elvanol 71-24," 0.075 g./l sc revi g had good color, brightness, and

us r. Lmwline, 15 Good product, nice brightness and luster,

some yellow stains. 2,6-lut1dene, 1.5 g./l "Elvanol 71-24, 0.038 g./l Fair product, semi-bright to bright, no

s ams. Nicotine-1, 1.12 g./l Elvanol 71-24, 0.075 g./l Screws had excellent brightness and luster,

. dark color, some yellow background. N cotme.1' 33 g ll do Exr zelle t color, brightness, and luster, no

s anung. Trimethylamme, 1,5 :1 ll -rln Scrtews were bright and lustrous, some s ams. do. Carbonate- 3-cyanopyiidine, 1.5 g.[l.- Elvenol 51-03," 0.15 g./1 Really excellent product, no staining.

Elvanol 7124Trademark for Du Ponts water-soluble polyvinyl alcohol. (Average molecular weight about 112,000). b Elvanol 5l-03Trademark for Du Ponts water-soluble polyvinylalcohol. (Average molecular weight about 16,800). c GEPVA- Glyceryl ether of polyvinyl alcohol. (Percentages indicate percent of alcohol group esterified with glycidol.) d HEPVAHydroxyethyl polyvinyl alcohol.

9 EXAMPLE v The relative'effectiveness of a brightening composition of the present invention and commercially available aldehyde based brighteners ina zinc cyanide plating solution was determined by experiments in which the consumption of the brightening agents per 100 ampere hours of electrolysis was measured. The experiments involved the continuous barrel plating of successive loads of screws over a period of several days. Only 0.55 gram per liter of a mixture (6:1 ratio) of nicotine acid N-oxide and a partially hydrolyzed (87-89%) polyvinyl alcohol having a molecular weight of about 16,800 was used. The plate was bright and lustrous. Using commercial brighteners, amounts ranging between 1.8 and 5.0 grams per liter (based on weight of active agents) per 100 ampere hours of electrolysis were required in order to maintain a similar level of brightness.

EXAMPLE VI This experiment demonstrates the stability of brightening compositions of this invention in the plating bath.

To a zinc cyanide plating solution was added 1.54 grams per liter of nicotinic acid N-oxide and 0.77 gram per liter of polyvinyl alcohol having an average molecular weight of about 112,000 (:1 ratio). After standing idle for nine days, the solution was used to plate screws in a barrel plating apparatus. The plated screws were very bright, lustrous, had a good color, and were of a comparable quality to screws plated from a freshly prepared plating solution.

In a comparative test, screws plated in a bath prepared with a commercial aldehyde based brightener having an initial efiiciency of about the same order as the nicotinic acid N-oxide brightener were very dull and had little luster after the bath had stood idle for nine days.

In practicing the present invention, other amine oxides may be substituted for those illustrated in the examples with similar results. Other soluble aliphatic amine oxides and aromatic amine oxides as well as mixed compounds may be used. Other polymeric materials such as glue, gum arabic, gum tragacan-th, gum ghat-ti, gum guaiac, as Well as protein substances such as egg albumin and milk protein hydrolysate may be used in combination with the amine oxides. While the preferred polymer is a polyvinyl alcohol having a molecular weight between about 15,000 and 17,000, those having molecular weigh-ts from 5,000 to over 100,000 provide beneficial results.

The plating baths exemplified may, of course, be modified to substitute for the sodium salts other salts, e.g., potassium and lithium compounds, which behave similarly in the bath. Also zinc oxide rather than zinc cyanide may be used. Details for preparing and using the baths are, of course, well known to those skilled in the art.

The primary advantage of the brightening agents of this invention resides in the excellent quality of bright zinc plate obtained. In addition, the amine oxides of this invention are very stable both in storage and in the plating bath during use and periods of non-use. Their rate of consumption is low thus making them economically attractive. Of particular importance is the fact that a formulation containing an amine oxide is usually equally effective both in still plating and in barrel plating operations.

A further advantage provided by the brightening agents of this invention is their superior throwing power. The combination of amine oxide and organic polymer gives bright, uniform coatings over a wide range of current densities, thus making it possible to coat intricately shaped articles.

The bright zinc deposits provided by the present invention are also responsive to treatment with protective agents. Excellent results are obtained using a variety of available commercial products such as chromate conversion coating preparations.

As many widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not to be limited to the specific embodiments thereof except as defined in the appended claims.

I claim:

1. An aqueous cyanide zinc electroplating bath containing as a brightening agent therefor an organic amine oxide having a z N O group, said amine oxide and the reduction product thereof obtained by removal of the oxygen from said EN'- O group being soluble in said bath, said amine oxide being present in a sufficient amount to yield a zinc electrodeposit of improved brightness.

2. The bath of claim 1 wherein said amine oxide is present in an amount from about 0.5 to 10 grams per liter of bath.

3. The bath of claim 2 wherein a bath-soluble organic polymer is present in an amount from about 0.1 to 10 grams per liter.

4. The bath of claim 3 wherein said amine oxide is a heterocyclic amine oxide and said organic polymer is a polyvinyl alcohol having a molecular weight from about 5,000 to 20,000.

5. The bath of claim 4 wherein said amine oxide is nicotinic acid N-oxide.

6. An aqueous cyanide zinc electroplating bath containing from about 0.1 to 10 grams per liter of an amine oxide having a N- O group and from about 0.02 to 10 grams per liter of a bath-soluble organic polymer selected from the group consisting of polyvinyl alcohol, polyethyleneimine, gelatin, and peptone, said amine oxide and the reduction product thereof obtained by removal of the oxygen from said EN)O group being soluble in said bath.

7. An aqueous cyanide zinc electroplating bath containing from about 0.1 to 10 grams per liter of nicotinic acid N-oxide and from about 0.05 to 0.5 gram per liter of a bath-soluble polyvinyl alcohol having a molecular weight from about 5,000 to 20,000.

8. The bath of claim 7 wherein said polyvinyl alcohol has a molecular weight of about 16,800.

9. An additive for an aqueous cyanide zinc electroplating bath consisting essentially of from about 1:1 to 50:1 parts by weight of an amine oxide having a EN O group and a bath-soluble organic polymer selected from the group consisting of polyvinyl alcohol, polyethyleneimine, gelatin, and peptone, said amine oxide and the reduction product thereof obtained by removal of the oxygen from said EN)O group being soluble in said bath.

10. The composition of claim 9 wherein said amine oxide is a heterocyclic amine oxide.

11. The composition of claim 10 wherein said amine oxide is nicotinic acid N-oxide and said organic polymer is polyvinyl alcohol having a molecular weight of about 16,800.

12. In the process for electrodepositing zinc from an aqueous cyanide plating bath the improvement comprising having present in said bath from about 0.1 to 10 grams per liter of an amine oxide containing a EN- O group, said amine oxide and the reduction product thereof obtained by removal of the oxygen from said EN O group being soluble in said bath.

13. The process of claim 12 wherein a bath-soluble organic polymer is added to said bath in an amount from about 0.02 to 10 grams per liter.

14. The process of claim 12 wherein said amine oxide is a heterocyclic amine oxide.

15. The process of claim 14 wherein said amine oxide is nicotinic acid N-oxide.

16. In a process for electrodepositing zinc from an aqueous cyanide plating bath, the improvement comprising having present in said bath from about 0.5 to 10 grams per liter of an amine oxide having a EN)0 group and from about 0.05 to 0.5 gram per liter of a bathsoluble organic polymer selected from the group consisting of polyvinyl alcohol, polyethyleneimine, gelatin, and peptone, said amine oxide and the reduction product thereof obtained by removal of the oxygen from said I-EN" O group being soluble in said bath.

17. In a process for electroplating zinc from an aqueous cyanide plating bath, the improvement comprising having present in said bath a brightening composition comprised of from about 0.5 to 10 grams per liter of nicotinic acid N-oxide and from about 0.05 to 0.5 gram per liter of a bath-soluble polyvinyl alcohol having a molecular weight from about 5,000 to 20,000.

References Cited by the Examiner UNITED STATES PATENTS Barrett et a1 20455 Bray et al 20455 Brown 204--49 Hoffman 20455 Foulke et a1. 20449 Jackson 20455 Passal 20455 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

1. AN AQUEOUS CYANIDE ZINC ELECTROPLATING BATH CONTAINING AS A BRIGHTENING AGENT THEREFOR AN ORGANIC AMINE OXIDE HAVING A $N-->O GROUP, SAID AMINE OXIDE AND THE REDUCTION PRODUCT THEREOF OBTAINED BY REMOVAL OF THE OXYGEN FROM SAID $N-->O GROUP BEING SOLUBLE IN SAID BATH, SAID AMINE OXIDE BEING PRESENT IN A SUFFICIENT AMOUNT TO YIELD A ZINC ELECTRODEPOSIT OF IMPROVED BRIGHTNESS. 